This application relates generally to gas turbine engine rotor assemblies and, more particularly, to bearing assemblies for gas turbine engine rotor assemblies.
Gas turbine engines typically includes a fan rotor assembly, a compressor, and a turbine. The fan rotor assembly includes a fan including an array of fan blades extending radially outward from a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan, and is supported longitudinally with a plurality of bearing assemblies. Bearing assemblies support the rotor shaft and typically include rolling elements located within an inner race and an outer race.
Additionally, at least some known bearing assemblies include a plurality of identical springs attached to the bearing outer race. The springs are spaced equally in a single row circumferentially around the rotor shaft to provide radial stiffness to the bearing and to center the outer race with respect to the support frame. A first end of the springs is attached to the bearing assembly outer race, and a second end of the springs is attached to a flange coupled to a support frame.
During operation, an unbalance within the engine may cause the engine rotor shaft to displace radially. The radial displacements of the shaft are transmitted to the bearing assembly. Because the springs are arranged in parallel the deflection of each spring is the same. A parallel configuration facilitates optimizing the weight of the bearing assembly by utilizing a shorter axial space. However, this configuration also reduces the spring bending stresses, thus increasing fatigue life. In this stress field, a generic single row configuration would require more springs, a greater material strength, greater cross-sectional inertia, and/or an increased spring length. As a result, a cost and weight of the bearing assembly would increase.
To minimize the effects of potentially damaging radial forces, the number of springs, the cross-sectional area, and the length of the springs are often increased. As a result, the cost and weight of the bearing assembly is also increased.
In an exemplary embodiment, a rotor assembly for a gas turbine engine includes a bearing centering sub-assembly that facilitates reducing radial forces transmitted to a bearing assembly. The bearing assembly supports a rotor shaft with a rolling element positioned radially inward from an outer race. The bearing centering sub-assembly is coupled to the outer race and includes a plurality of first springs and a plurality of second springs arranged in separate rows that extend circumferentially around the rotor assembly rotor shaft. The row of first springs is coupled between the row of second springs and the outer race such that each first spring is radially aligned with respect to each second spring. The row of second springs is coupled between the row of first springs and an engine support frame.
During normal operation, radial forces to the support frame are minimized with a bearing damper system including an oil plenum and springs. The oil plenum is formed by a radial gap extending between the outer race and the support frame. The spring stiffness dictates a natural frequency for the rotor, and the oil plenum controls the frequency response or radial deflection. The springs are beams arranged circumferentially around the rotor shaft, and center the outer race with respect to the support frame, such that the oil plenum is defined.
A plurality of beam properties determines a spring stiffness of each beam. More specifically, a cross sectional inertia, a material modulus of elasticity, a length of each beam determines the spring stiffness. In addition, as the number of beams is increased, the spring stiffness is increased. The spring stiffness is selected such that as the rotor deflects with respect to the support frame, the oil plenum dampens radial forces induced to the support frame.
During normal rotor unbalances, spring stresses are a function of length. The parallel configuration reduces bending stresses by approximately fifty percent in comparison the single row configuration. In addition, the parallel configuration also increases fatigue life in comparison the single row configuration.
During high rotor unbalance, the outer race spring deflection is significant to bottom the radial gap in the oil plenum. A circumferential force is created on the outer race springs yielding the beams in bending. Because the springs are parallel, all springs are reduced in length an equal amount. Accordingly, a net axial translation or displacement of the bearing assembly rolling element is approximately zero.